In communications and radar systems, it is sometimes important to process signals having a desired range delay while rejecting signals having an undesired range delay. The out-of-range rejection (ORR) performance, synonymously referred to herein as the autocorrelation performance, is a measure of such a systems ability to accomplish this requirement. Pseudorandom Noise (PN) codes systems are often used to achieve acceptable ORR performance while maintaining low peak transmit power requirements. Such PN coded systems are classified as spread spectrum systems because of the signal bandwidth spreading effected by the PN encoding. Direct sequence multiple access modulation protocols such as the direct sequence Code Division Multiple Access (CDMA) protocol widely used in spread spectrum communications is an example of a communications system to which this invention may apply. PN coded radar systems in common use are examples of radar systems to which this invention may apply.
Most PN coded systems to which this invention apply modulate the PN code onto a carrier frequency prior to transmission, and subsequently demodulate the PN code in the receive path in a correlation process. Conventional pseudorandom noise (PN) systems use a biphase modulator to modulate the characteristics of a maximal length binary code sequence onto the continuously transmitted carrier frequency. The biphase modulator has two phase states which are nominally separated 180 degrees in phase, and which are selected in response to the two states of the binary code sequence. In such a conventional PN system, the ORR performance is limited, it being proportional to the square of the PN code length.
In designing a PN coded radar system there is a system tradeoff to be made between the various waveform parameters including: the PN code length, PN code chip width, Doppler frequency bandwidth, and transmitter center frequency. The PN code length establishes the system's ORR performance, the PN code chip width establishes the system's range resolution, the Doppler frequency bandwidth and the transmitter center frequency establish the maximum closing velocity. For a given range resolution, closing velocity and transmitter center frequency, the PN code length and consequently the ORR performance is limited. Thus, this system tradeoff often results in a compromise between conflicting system requirements.
In designing a PN coded communications system there is a similar system tradeoff to be made between these various waveform parameters. The PN code chip width establishes the system's range resolution for distinguishing signals from a desired range while rejecting signals from an unwanted range, and also establishes the transmission bandwidth. A longer PN code length is often desired to improve the ORR performance. However, for a given chip width, increasing the PN code length to improve the ORR performance will decrease the usable signal information bandwidth. In a direct sequence multiple access system, for example, the improved autocorrelation performance is desired because this decides how well the locally generated code signal can be synchronized and locked to the received signal. Also, improved autocorrelation performance enhances the ability of a direct sequence multiple access system to combat the effects of multipath interference and channel fading.
What is needed is a means for providing a significant improvement in the ORR performance while maintaining the same or shorter PN code length. This would provide greater flexibility in selecting the PN modulation waveform parameters, and would allow for the simultaneous improvement of other performance parameters.
In a spread spectrum PN radar system, for example, such a means would allow an increase in maximum target engagement velocity and/or an increase in the transmit frequency to permit improvements in antenna performance, while simultaneously improving the ORR performance. It would also provide a capability to detect very small targets in the presence of large out-of-range clutter return using a shorter PN code length.
In a spread spectrum communications system such a means would allow simultaneous improvements in signal information bandwidth and autocorrelation performance while maintaining the system range resolution and transmission bandwidth. This will enable performance improvements in the presence of multipath interference and channel fading, and also enable easier synchronization and locking of the received signal to the locally generated PN code generator.